1. Field of the Invention
The present invention relates generally to positioning systems and methods, and more particularly to positioning systems which control the movement of and therefore position a substrate such as a semiconductor wafer which includes many integrated circuits arranged on the substrate.
2. Background of the Invention
Positioning systems are used in various aspects of semiconductor fabrication. For example, photolithographic systems and wafer probers and other semiconductor integrated circuit fabrication tools often use a wafer positioning system. Most wafer positioning or manipulator stages are based on combinations of linear motions along orthogonal directions. An example of a conventional linear motion system in a wafer prober will now be described by referring to FIGS. 1 and 2.
A semiconductor wafer normally includes many integrated circuits which are formed in a lattice of devices or integrated circuits. On each integrated circuit, there are a plurality of bonding pads which are used to connect the integrated circuit to external circuitry in order to allow the integrated circuit (IC) to function. Since the packaging of each IC is somewhat expensive, it is desirable to test each IC before packaging to avoid packaging defective IC's. This process of testing devices before packaging is referred to as the sort process. This process involves connecting a device referred to as a probe card to a special tester. FIG. 1 shows an example of a wafer prober system 10 which includes a generalized example of a probe card 17 which is mounted on the support 16. The probe card 17 includes a collection of pins 18 which stand in for the normal pins and wire leads of a package device. These pins 18 are made to come into electrical contact with the bonding pads 14 of at least one integrated circuit on the semiconductor wafer 12. The semiconductor wafer 12 rests on a wafer chuck 11, which may be referred to as a wafer holding platform. The wafer prober system 10 positions each IC on the wafer with respect to the probe card so that the appropriate pins 18 on the probe card contact the appropriate pads 14 for a particular IC on the wafer 12.
As the state of the semiconductor processing art progresses, wafers get larger, die geometries get smaller, the number of pads on each die increase, and the size of each pad decreases. Hence the alignment accuracy and speed requirements for a wafer prober become more stringent. These stringent requirements place great demands on the manipulator or positioning stages used in a wafer prober. These positioning stages or systems work in conjunction with modern vision systems which utilize cameras, such as cameras 15 and 20 which are designed to view the probe card and the wafer, respectively, in order to attempt to accurately align an IC on a wafer with respect to the contact pins on a probe card.
FIG. 2 shows an example of a conventional wafer positioning system which may be used in a wafer prober or other semiconductor fabrication tool. It will be appreciated that another conventional wafer positioning system may use a Sawyer motor stage; see for example, U.S. Pat. No. 4,455,512. For wafer probers, these positioning systems must provide four axes (X,Y,Z,.theta.) of motion. The usual implementation includes an X,Y stage for positioning in X,Y and an independent Z stage and an independent .theta. stage. FIG. 2 shows a wafer positioning system 30 which utilizes a conventional rectilinear X,Y stage. A wafer chuck 11 is positioned on top of the Y motion stage 31 which rides along the guide rails 32a and 32b to provide translation in the Y direction. The wafer chuck 11 may include a rotary motor positioned on top of the Y motion stage 31 and below the chuck 11 in order to provide .theta. motion for the wafer chuck 11. The X motion stage 33 rides along guide rails 34b and 34a in order to provide translation along X. The wafer 40 is positioned on top of the wafer chuck 11 and is typically held in place on top of the chuck by a vacuum generated under the surface of the wafer by the chuck. A separate Z stage provides movement up and down in Z by either increasing the distance between the wafer chuck 11 and the Y stage 31 or by moving the entire X and Y stages 31 and 33 (with their associated guide rails) up and down in the Z direction.
Wafer positioning stages which are based upon combinations of linear motions along orthogonal directions, such as wafer positioning system 30, are expensive and difficult to manufacture when very precise positioning resolution is required. As geometries on an integrated circuit decrease, positioning resolution must also improve to the point that a positioning resolution of a few microns is often required. Hence, it is desirable to provide a wafer positioning system which is inexpensive to manufacture and also provides high positioning resolution.